The present invention relates to communication systems and in particular to the detection of an impulse position of a multipath channel in a noisy environment of a wireless communication system.
Impulse response measurements of a communication channel provide important information for the design of a communication system. Since the impulse response defines a system, it can be used to simulate a communication channel in order to ascertain the effects of the channel on a signal. This can be used to simulate a received signal as the transmitted is distorted through the channel by convolving the impulse response with the transmitted signal.
In a conventional spread-spectrum communication system, the information signal with a pseudo-random number (PN, also referred to a pseudo-noise) sequence to produce the transmitted signal. The received signal passed through a match filter to produce a matched signal. The matched signal is correlated with a local copy of the PN sequence to produce a correlated signal. Ideally, the matched signal comprises only the line-of-sight signal (i.e., the signal traveling along the straight-line path from transmitter to receiver), from which the impulse response of the channel over which the signal was transmitted can be determined.
The position of the channel impulse response can be used to determine the line-of-sight distance between the receiver and transmitter. The reality, however, is there are usually a multitude of objects (e.g., building, natural obstacles such as trees, hills, and so on) from which the transmitted signal can be reflected. This results in the phenomenon known as “multipath,” whereby a signal can travel more than one route between the transmitter and receiver by virtue of reflections from the objects. The receiver can “see” many copies of the transmitted signal, including the line-of-sight signal and the reflected signals. The resulting impulse response therefore represents the combined effect of the multipath signals, masking the response of the line-of-sight signal.
Identifying the line-of-sight signal is made more difficult if there is an object between the transmitter and receiver, thus attenuating the signal strength. Also, at some frequencies the signal strength may attenuate or cancel due to of out-of-phase signals, or even increase if two reflected signals are in phase. The resulting impulse response would consist of multiple impulse positions of varying amplitude, thus making it difficult to identify the position of the channel impulse response of the line-of-sight signal. Another real world consideration is that if there is relative motion between the transmitter and receiver, then the path lengths of the reflected signals will vary. This produces fluctuations in amplitude and relative phase among the signals, thus making it difficult to apply conventional thresholding techniques.
It can be appreciated therefore that opportunity exists for improvements in making a quick determination of impulse position in a multipath channel.